1. Field of the Invention
The present invention relates to a variable gain amplifier having a low NF (noise figure), used for a front-end of a receiving apparatus.
2. Description of the Related Art
A configuration of a conventional variable gain amplifier is shown in FIG. 9. A variable gain amplifier 110 is provided with a plurality of variable gain amplifier circuits called a first variable gain amplifier circuit 102, a second variable gain amplifier circuit 104, and a third variable gain amplifier circuit 106, a plurality of attenuation circuits called a first attenuation circuit 103 and a second attenuation circuit 105, and a gain control section 108.
An input of the first variable gain amplifier circuit 102 is connected to an RF input terminal 1 and an output thereof is connected to an RF output terminal 6. An input of the second variable gain amplifier circuit 104 is connected to the RF input terminal 1 through the first attenuation circuit 103 and an output thereof is connected to the RF output terminal 6. An input of the third variable gain amplifier circuit 106 is connected to the input of the second variable gain amplifier circuit 104 through the second attenuation circuit 105 and an output thereof is connected to the RF output terminal 6.
The gain control section 108 varies a gain of the first variable gain amplifier circuit 102, a gain of the second variable gain amplifier circuit 104, and a gain of the third variable gain amplifier circuit 106 according to a gain control voltage inputted into a gain control voltage input terminal 7.
In order to vary the gain of the variable gain amplifier circuit, means for varying a bias current of the variable gain amplifier circuit is often used.
Next, an operation of the variable gain amplifier 110 will be described using FIG. 10. FIG. 10 shows the gain of each signal path within the variable gain amplifier 110 versus the gain control voltage. Symbol G1 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is amplified by the first variable gain amplifier circuit 102 and is subsequently outputted to the RF output terminal 6. Symbol G2 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is attenuated by the first attenuation circuit 103 and is further amplified by the second variable gain amplifier circuit 104, and is subsequently outputted to the RF signal output terminal 6. Symbol G3 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is attenuated by the first attenuation circuit 103, is further attenuated by the second attenuation circuit 105, and is further amplified by the third variable gain amplifier circuit 106, and is subsequently outputted to the RF signal output terminal 6. Symbol G4 indicates the gain of the variable gain amplifier 110.
When the gain of the variable gain amplifier 110 becomes the maximum, the gain control section 108 operates so that the gain of the first variable gain amplifier circuit 102 may become the maximum, the gain of the second variable gain amplifier circuit 104 may become the minimum, and the gain of the third variable gain amplifier circuit 106 may become the minimum. When the gain control voltage changes so that the gain of the variable gain amplifier 110 may be reduced, the gain control section 108 operates so that the gain of the first variable gain amplifier circuit 102 may be reduced first and the gain of the second variable gain amplifier circuit 104 may be increased. When the gain control voltage changes so that the gain of the variable gain amplifier 110 may be further reduced, the gain control section 108 operates so that the gain of the first variable gain amplifier circuit 102 may be further reduced, the gain of the second variable gain amplifier circuit 104 may change to be reduced after increasing to the maximum, and the gain of the third variable gain amplifier circuit 106 may be increased. When the gain of the variable gain amplifier 110 becomes the minimum, the gain of the first variable gain amplifier circuit 102 becomes the minimum, the gain of the second variable gain amplifier circuit 104 becomes the minimum, and the gain of the third variable gain amplifier circuit 106 becomes the maximum.
As the conventional variable gain amplifier, there has been a variable gain amplifier described in Japanese Unexamined Patent Publication (Kokai) No. 2003-60457. FIG. 11 shows a variable gain amplifier 117 described in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 2003-60457. In FIG. 11, the same symbol is given to the same configuration as that shown in FIG. 9, and description there of will be omitted. The variable gain amplifier 117 is provided with a plurality of variable gain amplifier circuits called a variable gain amplifier circuit 111, a variable gain amplifier circuit 112, and a variable gain amplifier circuit 113, whose maximum gains are different form each other. Respective variable gain amplifier circuits are inserted in parallel between the RF input terminal 1 and the RF output terminal 6.
A gain of the variable gain amplifier circuit 111 is varied by a gain control voltage and a bias current set in a current control circuit 116 according to the gain control voltage. A gain of the variable gain amplifier circuit 112 is varied by a voltage obtained by shifting the gain control voltage in a voltage shift circuit 114 by a predetermined voltage, and a bias current set by the current control circuit 116 according to the gain control voltage. A gain of the variable gain amplifier circuit 113 is varied by a voltage obtained by further shifting the voltage, which is obtained by shifting the gain control voltage in the voltage shift circuit 114 by the predetermined voltage, in a voltage shift circuit 115 by a predetermined voltage, and a bias current set by the current control circuit 116 according to the gain control voltage.
Next, an operation of the variable gain amplifier 117 will be described using FIG. 12. FIG. 12 shows the gain of each signal path within the variable gain amplifier 117 versus the gain control voltage. Symbol G1 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is amplified by the variable gain amplifier circuit 111, and is subsequently outputted to the RF output terminal 6. Symbol G2 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is amplified by the variable gain amplifier circuit 112, and is subsequently outputted to the RF output terminal 6. Symbol G3 indicates the gain of the signal path in which the RF signal inputted into the RF input terminal 1 is amplified by the variable gain amplifier circuit 113, and is subsequently outputted to the RF output terminal 6. Symbol G4 indicates the gain of the variable gain amplifier 117.
When the gain of the variable gain amplifier 117 becomes the maximum, the current control circuit 116 sets the bias current of the variable gain amplifier circuit 111 to be maximum and the bias currents of the variable gain amplifier circuits 112 and 113 to be minimum so that the gain of the variable gain amplifier circuit 111 may become the maximum, the gain of the variable gain amplifier circuit 112 may become the minimum, and the gain of the variable gain amplifier circuit 113 may become the minimum.
When the gain control voltage changes so that the gain of the variable gain amplifier 117 may be reduced, the current control circuit 116 sets the bias current of the variable gain amplifier circuit 111 to be reduced and the bias current of the variable gain amplifier circuit 112 to be increased so that the gain of the variable gain amplifier circuit 111 may be reduced first and the gain of the variable gain amplifier circuit 112 may be increased.
Further, when the gain control voltage changes so that the gain of the variable gain amplifier 117 may be reduced, the current control circuit 116 sets the bias current of the variable gain amplifier circuit 111 to be further reduced, the bias current of the variable gain amplifier circuit 112 to be changed to be reduced after increasing to the maximum, and the bias current of the variable gain amplifier circuit 113 to be increased so that the gain of the variable gain amplifier circuit 111 may be further reduced, the gain of the variable gain amplifier circuit 112 may change to be reduced after increasing to the maximum, and the gain of the variable gain amplifier circuit 113 may be increased.
Further, when the gain control voltage changes so that the gain of the variable gain amplifier 117 may be reduced, the current control circuit 116 sets the bias current of the variable gain amplifier circuit 111 to be further reduced, the bias current of the variable gain amplifier circuit 112 to be further reduced, and the bias current of the variable gain amplifier circuit 113 to be changed to be reduced after increasing to the maximum so that the gain of the variable gain amplifier circuit 111 may be further reduced, the gain of the variable gain amplifier circuit 112 may be further reduced, and the gain of the variable gain amplifier circuit 113 may be reduced after increasing to the maximum.
In the above-mentioned conventional variable gain amplifier 110, however, when a wide gain variable range is intended to be taken, there has been a problem that a circuit scale including the control circuit has been increased when securing an excellent linearity.
As described above, respective variable gain amplifier circuits 111 through 113 composing the variable gain amplifier 110 control the gains by controlling the bias currents in many cases, but since the gains of the variable gain amplifier circuits 111 through 113 are reduced, linearities of the variable gain amplifier circuits 111 through 113 deteriorate when reducing the bias currents of the variable gain amplifier circuits.
When large attenuation amounts of the first attenuation circuit 103 and the second attenuation circuit 105 are taken in order to take the wide gain variable range of the variable gain amplifier 110, a ratio for the output of the first variable gain amplifier circuit 102 to contribute to the RF output of the variable gain amplifier 110 becomes higher than a ratio for the output of the second variable gain amplifier circuit 104 to contribute to the RF output of the variable gain amplifier 110 or a ratio for the output of the third variable gain amplifier circuit 106 to contribute to the RF output of the variable gain amplifier 110, even in a situation where, for example, the bias current of the first variable gain amplifier circuit 102 is reduced, the gain thereof is decreased, and the linearity is deteriorated, and thus the linearity of the variable gain amplifier 110 will deteriorate. In order to suppress the deterioration of the linearity and to extend the gain variable range of the variable gain amplifier 110, it is necessary to increase the variable gain amplifier circuits in the number of stages, resulting in a large circuit scale.
Also in a case of the variable gain amplifier 117, when large maximum gain differences among the variable gain amplifier circuit 111, the variable gain amplifier circuit 112, and the variable gain amplifier circuit 113 are taken in order to take the wide gain variable range of the variable gain amplifier 117, the linearity of the variable gain amplifier 117 will deteriorate. In order to suppress the deterioration of the linearity and to extend the gain variable range of the variable gain amplifier 117, it is necessary to increase the variable gain amplifier circuits in the number of stages, resulting in a large circuit scale